High-temperature kiln



Dec. 28, 1965 J, H. DOWNING ETAL 3,226,465

HIGH-TEMPERATURE KILN Filed May 31, 1963 INVENTORS JAMES H. DOWNING ERNEST KOERNER JAMES E. WELLS m ATTORNEY United States Patent 3,226,465 HlGH-TEMPERATURE KILN James H. Downing, Qlarence, N.Y., Ernest L. Koerner, Bridgeton, Mo., and James E. Wells Ill, Kenmore, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed May 31, 1963, Ser. No. 284,519 4 Claims. (Ci. 1327) This invention relates to metallurgical kilns. More particularly, the present invention relates to an inductively heated rotary kiln which provides high temperatures and a uniform, controlled atmosphere within the kiln.

Rotary kilns are widely used in the manufacture of cement and lime and for treating ores. Such kilns are heated internally by the combustion of carbonaceous gas, oil, or powdered coal and the control of the atmosphere within the kilns, using these and like materials, is extremely dilficult at best.

When an oxidizing atmosphere is desired in the kiln, an excess of air can be supplied, and when a reducing atmosphere is desired an excess of fuel can be provided. However, due to the difiiculty of obtaining complete intermixing of fuel and air, particularly with solid and liquid fuels, the kiln atmosphere varies between oxidizing and reducing along its length even with the most precise control of the fuel-air ratio.

The kiln temperatures that may be obtained, by combustion of a fuel-air mixture, are limited to about 1400 to 1500 C. Although somewhat higher temperatures may be achieved using oxygen enriched air, this technique practically precludes the possibility of obtaining a neutral or reducing atmosphere in the kiln. Since reducing atmospheres at high temperatures can be highly advantageous in various industrial applications, the inability of previously available kilns to provide such conditions represents a continuing problem.

It is therefore an object of the present invention to provide a rotary kiln capable of producing high temperatures and a uniform atmosphere.

Other objects will be apparent from the following description and claims taken in conjunction with the drawing in which The single figure shows an elevation view, partly in section, of an embodiment of the kiln of the present invention.

A rotary kiln in accordance with the present invention comprises an inductively heated and insulated graphite tube which is rotatably driven and sealed from the ambient atmosphere.

With reference to the drawing, the single figure shows a graphite tube 1 surrounded by an outer concentric tube 3 which is formed of a material which is not electrically conductive and which is a good heat insulator, e.g. asbestos bonded with mineral cement. Other nonconductive materials having suitable strength can also be used.

The space between the concentric tubes is filled with amorphous carbon 5 and sealed by end rings 7 which can be formed of porous carbon.

The concentric tube arrangement constitutes the body of the kiln which is rotatably supported by sealably grooved steel end-band supports 9. Rollers 11, driven by a suitable drive mechanism 13, engage the steel endband supports 9 and impart rotation to the kiln body. A concentric tubular steel extension 15, sealably attached to the graphite tube 1 adjacent its inlet end, by way of end band supports 9, is slidably engaged in circular groove 17 and bears against graphite plate 19. Graphite plate 19 is an integral part of the gas tight housing indicated at 21 and surrounds the opening 23 in the housing through which the graphite tube 1 extends. Seal ring 3,22%,455 Patented Dec. 28, 1965 25, is fixedly attached to graphite plate 19 and is slidably engaged against steel tube entension 15 to provide a substantially gas-tight seal between the housing 21 and the kiln body. Housing 21 encloses the feed end of the kiln and the charge material enters the kiln from gas tight hopper 27 through conduit 29 which communicates through a gas-tight seal 31 with the housing. End cap 33, on graphite tube 1 is merely to minimize spillage of the charge and is not essential. Any spillage is collected in gas-tight collector 35. The conduit 36 communicating with collector 35 is provided with a gas-tight seal at the position where it passes through the housing.

The housing 21 is provided with a gas exit 37 for removal of gases from the kiln and the gas exit 37 is connected to a cyclone 39 and exhaust trap 41 as shown.

At the opposite discharge end of the kiln a similar gas-tight housing 21' is provided having a gas tight connection with kiln discharge receiver 43. Steel tube extension 15', sealably attached to graphite tube 1, by way of end band supports 9, is rotatably engaged with graphite plate 19' and in sealing relations with seal ring 25' in the same manner as at the inlet end of the kiln. The housing 21' is additionally provided with gas inlet ports 45 and 47 for the introduction of non-oxidizing gases into the kiln. Also, sights 49 and 51 are provided in housing 21' to permit observation of the interior of the kiln and product discharge. Water sprays 53 provide cooling of the end ring at the vicinity of the outlet of the kiln. Within the graphite tube 1 porous carbon disks 55 and 57 are fixedly positioned adjacent the kiln inlet and outlet ends, respectively. These disks have spiral grooves 59 which serve to advance the charge from the inlet to outlet ends of the kiln upon rotation of the graphite tube 1. The disks can be coated with tungsten carbide for better wear resistance. The opposing surfaces 61 and 63 of the porous carbon disks 55 and 57 act as heat reflecting surfaces.

Also, the disks are provided with apertures 65 and 67 to permit passage of gas through the kiln and to permit visual inspection through sight 51.

The heating of the kiln is accomplished by induction coil 69 which surrounds the kiln and in which the kiln is free to rotate. Alternating current source 71 is connected to coil 69 and supplies the electrical energy for the development of induction heating in graphite tube 1.

Tie rods 75 hold the gas-tight housings 21 and 21 in sealed relation with the graphite tube, in spite of lengthwise expansion, by means of spring 77 which bears against plate 79 through which the tie rod passes. Plate 79, and plate 81, which engage tie rod 75, are integral with housings 21 and 21 respectively.

In the operation of the apparatus of the present invention, a charge material, e.g. anthracite coal, is introduced into tube 1 from feed hopper 27 and the rotation of the kiln is adjusted to provide a satisfactory residence time in the kiln, usually 30 to 60 minutes for the production of graphitic carbon from coal. During the passage of material through the kiln, a non-reactive atmosphere is maintained in the kiln by introducing argo gas or other inert gas through inlet 47.

The induction coil 69, energized by alternating current source 71, provides substantially uniform heating in the portion of the graphite tube surrounded by the coil; for the graphitization of coal, the temperature provided in the kiln is between 1800 C. and 2000 C.

The spiral grooved disks 55 and 57 assist in the transfer of material through the kiln to the collector 43. Also, the heat reflecting surfaces 61 and 63 of the disks 55 and 57, and the insulating material surrounding graphite tube 1, Le. the amorphous carbon 5 and tube 3, conserve the heat developed in the kiln and permit the development of high temperatures without detrimental effect on the other parts of the kiln apparatus.

The charge material exiting the kiln is collected in receiver 43 as previously mentioned. When desired, the product can be removed from the discharge receiver without the admission of air into the kiln, by adjustment of volve assembly 73.

Both the valve assembly 73 and receiver 43 are provided with gas inlets '75 and 77 for the introduction of non-oxidizing gases for purging the system.

The following examples are provided to further illustrate the present invention.

EXAMPLE I A rotary kiln of the type shown in the drawing was used to produce graphitic carbon from green anthracite coal.

The graphite tube of the kiln was /1 thick, 9 ft. long with a 6 inch inner diameter. The tube was inclined from inlet to outlet at an angle of about 1-2 degrees. The surrounding layer of amorphous carbon was 3 /2 inch thick and the outer tube, A2 inch thick, was formed of transite.

The green anthracite coal, sized 8 by 28 mesh (Tyler Screen Series), was passed through the kiln which was rotated at 2 to 4 rpm. An argon atmosphere was provided in the kiln.

The retention time of the charge material in the kiln was 30 to 60 minutes and the temperataure in the kiln was 1870 C. to 2015 C. The heating of the kiln was provided by a turn induction coil which was energized by 800 volts 300 cycles per second.

A sample analysis in Weight percent of the green coal, the calcined product, and the fume solids collected from the exhaust gas is shown in Table I.

Table II shows various operating conditions and the resistance indices of the graphite products.

A low resistance index (less than 2) is desirable when the material is to be used in the manufacture of electrodes for electrolytic aluminum cells.

Table II Kiln Retention Kiln Av. Run N0. Rotation, 'lnne, Temp, Resistance rpm. niin. C. Index The resistance index of the product obtained in gasfired rotary kilns averages between 3.75 and 4.75.

EXAMPLE II The same kiln as in Example I was used to prepare high-purity tantalum carbide. For this purpose, tantalum oxide from two separate sources was used. One lot of tantalum oxide was prepared by oxidizing scrap tantalum metal and the other by chemical methods from ore. Powdered graphite was used as reductant with both lots.

A The analysis in weight percent of the starting materials is given in Table III.

Table III Acheson 1 Lot 1 Oxide Lot 2 Oxide Graphite, Grade 38 Ash 1 Trademark of Union Carbide Corporation.

2 N 0t detected.

Fifty pounds of a mix containing stoichiometric carbon and lot 1 oxide and 325 pounds of a mix containing stoichiometric carbon and lot 2 oxide were pelletized. The pellets were passed through the kiln and reacted to The analyses of the products are given in Table V.

Table V Run Percent Percent Percent Percent Total 0 Free 0 O N An argon atmosphere was provided in the kiln. Unless the atmosphere is non-oxidizing, in the kiln, the carbide conld not be quantitatively formed.

The pellets recovered from each run were dense and hard and very few fines were produced-less than 5 percent minus 6 mesh, and 2 percent minus 6 mesh, for runs A and B respectively. (Tyler Screen Series.)

What is claimed is:

1. A rotary kiln comprising a first inner graphite tube adapted to contain a charge of solid material having a charge inlet end and an outlet end; a second outer tube of non-conductive material arranged concentric with and spaced from the first tube and in fixed relation therewith, the space between the first and second tubes being closed and substantially filled with amorphous carbon; means communicating with said graphite tube in a substantially gas-tight connection for introducing charge material into said graphite tube at the inlet end thereof substantially without the admission of air to the graphite tube; means for withdrawing gases from the graphite tube at its inlet end; means for recovering solid material from the graphite tube at its outlet end substantially without the admission of air to the graphite tube; means for axially rotating the graphite tube; and means for electrically heating the graphite tube.

2. A rotary kiln comprising an inner graphite tube adapted to contain a charge of solid material having a charge inlet end and an outlet end; an outer tube of nonconductive material arranged concentric with and spaced from the inner graphite tube and in fixed relation thereto;

the space between the inner and outer tubes being closed and substantially filled with amorphous carbon; a first substantially gas-tight housing enclosing the inlet end of said graphite tube and arranged to permit rotation of said graphite tube in said housing; a second substantially gastight housing enclosing the outlet end of said graphite tube and arranged to permit rotation of said graphite tube in said housing; feeding means sealed from the ambient atmosphere communicating with the graphite tube through said first housing for introducing charge material into said graphite tube; gas exhaust means communicating with said graphite tube through said first housing; rotating means coupled to said graphite tube exterior said housing; collecting means sealed from the ambient atmosphere communicating with the graphite tube through said second housing to receive the solid material exiting the graphite tube; electric coil means surrounding said outer tube and adapted to provide inductive heating in said graphite tube; and a source of alternating electrical energy connected to said coil.

3. A rotary kiln comprising an inner graphite tube adapted to contain a charge of solid material having a charge inlet end and an outlet end; an outer tube of nonconductive material arranged concentric with and spaced from the inner graphite tube and in fixed relation thereto; the space between the inner and outer tubes being closed and substantially filled with amorphous carbon; a first cylindrical steel tube extension concentric with said graphite tube and sealably attached thereto adjacent its inlet end; a second cylindrical steel tube extension concentric with said graphite tube and sealably attached thereto adjacent its outlet end; a first substantially gas-tight housing positioned adjacent the inlet end of said graphite tube and communicating therewith through an opening in said first housing; a first graphite plate integral with said first housing surrounding said opening and having a circular groove surrounding said opening, said first graphite plate being slidably engaged with said first steel tube extension at said circular groove; a seal ring in contact with said first graphite plate and said first steel tube extension adjacent said circular groove to provide a substantially gastight seal between said first graphite plate and said first steel tube extension; a second substantially gas-tight housing positioned adjacent the inlet end of said graphite tube and communicating therewith through an opening in said second housing; a second graphite plate integral with said second housing surrounding said opening and having a circular groove surrounding said opening, said second graphite plate being slidably engaged with said second steel tube extension at said circular groove; a seal ring in contact with said second graphite plate and said second steel tube extension adjacent said circular groove to provide a substantially gas-tight seal between said second graphite plate and said second steel tube extension; feeding means sealed from the ambient atmosphere communicating with the graphite tube through said first housing for introducing charge material into said graphite tube; gas exhaust means communicating with said graphite tube through said first housing; rotating means coupled to said graphite tube exterior said first housing; collecting means sealed from the ambient atmosphere communicating With the graphite tube through said second housing to receive the solid material exiting the graphite tube; electric coil means surrounding said outer tube and adapted to provide inductive heating in said'graphite tube; and a source of alternating electrical energy connected to said coil.

4. A rotary kiln comprising an inner graphite tube adapted to contain a charge of solid material having a charge inlet end and an outlet end; an outer tube of nonconductive material arranged concentric with and spaced from the inner graphite tube and in fixed relation thereto; the space between the inner and outer tubes being closed and substantially filled with amorphous carbon; a first cylindrical steel tube extension concentric with said graphite tube and sealably attached thereto adjacent its inlet end; a second cylindrical steel tube extension concentric with said graphite tube and sealably attached thereto adjacent its outlet end; a first substantially gas-tight housing positioned adjacent the inlet end of said graphite tube and communicating therewith through an opening in said first housing; a first graphite plate integral with said first housing surrounding said opening and having a circular groove surrounding said opening, said first graphite plate being slidably engaged with said first steel tube extension at said circular groove; a seal ring in contact with said first graphite plate and said first steel tube extension adjacent said circular groove to provide a substantially gas-tight seal between said first graphite plate and said first steel tube extension; a second substantially gas-tight housing positioned adjacent the outlet end of said graphite tube and communicating therewith through an opening in said second housing; a second graphite plate integral with said second housing surrounding said opening and having a circular groove surrounding said opening, said second graphite plate being slidably engaged with said second steel tube extension at said circular groove; a seal ring in contact with said second graphite plate and said second steel tube extension adjacent said circular groove to provide a substantially gas-tight seal between said second graphite plate and said second steel tube extension; feeding means sealed from the ambient atmosphere communicating with the graphite tube through said first housing for introducing charge material into said graphite tube; gas exhaust means communicating with said graphite tube through said first housing; rotating means coupled to said graphite tube exterior said housing; collecting means sealed from the ambient atmosphere communicating with the graphite tube through said second housing to receive the solid material exiting the graphite tube; a first spiral grooved, apertured disk fixedly positioned and axially aligned in said graphite tube adjacent the inlet end thereof and a second spiral grooved apertured disk fixedly positioned and axially aligned in said graphite tueb adjacent the outlet end thereof, the opposing surfaces of the disks being formed of heat reflecting material and said spiral grooved in said disks being arranged to advance the charge material through the graphite tube from the inlet end to the outlet end, the apertures in the disks being arranged to permit the passage of gas through said graphite tube; electric coil means surrounding said outer tube and adapted to provide inductive heating in said graphite tube; and a source of alternating electrical energy connected to said coil.

References Cited by the Examiner UNITED STATES PATENTS 1,601,222 9/1926 Naugle 137 1,763,229 6/1930 Fourment 21910.51 X 2,621,218 12/1952 Juckniess 13--7 2,729,556 1/ 1956 Fontana 13--27 RICHARD M, WOOD, Primary Examiner, 

1. A ROTARY KILN COMPRISING A FIRST INNER GRAPHITE TUBE ADAPTED TO CONTAIN A CHARGE OF SOLID MATERIAL HAVING A CHARGE INLET END AND AN OUTLET END; A SECOND OUTER TUBE OF NON-CONDUCTIVE MATERIAL ARRANGED CONCENTRIC WITH AND SPACED FROM THE FIRST TUBE AND IN FIXED RELATION THEREWITH, THE SPACE BETWEEN THE FIRST AND SECOND TUBES BEING CLOSED AND SUBSTANTIALLY FILLED WITH AMORPHOUS CARBON; MEANS COMMUNICATING WITH SAID GRAPHITE TUBE IN A SUBSTANTIALLY GAS-TIGHT CONNECTION FOR INTRODUCING CHARGE MATERIAL INTO SAID GRAPHITE TUBE AT THE INLET END THEREOF SUBSTANTIALLY WITHOUT THE ADMISSION OF AIR TO THE GRAPHITE TUBE; MEANS FOR WITHDRAWING GASES FROM THE GRAPHITE TUBE AT ITS INLET END; MEANS FOR RECOVERING SOLID MATERIAL FROM THE GRAPHITE TUBE AT ITS OUTLET END SUBSTANTIALLY WITHOUT THE ADMISSION OF AIR TO THE GRAPHITE TUBE; MEANS FOR AXIALLY ROTATING THE GRAPHITE TUBE; AND MEANS FOR ELECTRICALLY HEATING THE GRAPHITE TUBE. 